1. Field of the Invention
The present invention generally relates to rotary magnetic couplings, also known as magnetic drives or magnetic clutches, and more particularly to an assembly and method for pre-stressing a canister in a magnetic coupling device where the canister is positioned between an inner magnet assembly and an outer magnet assembly to statically seal a fluid chamber.
2. Discussion of the Prior Art
In many rotating devices such as pumps, it is desirable to avoid potential seal leakage by not using seals in conjunction with rotating parts. Accordingly, it has become more common to employ a magnetically coupled drive system to eliminate the need for seals along rotating surfaces. While such devices may still employ static seals, because of their lack of dynamic or rotational seals, they have become known as “sealless”. Indeed, magnetic drive structures have been used in the design of various pumps, including positive displacement gear pumps. Magnetic couplings or drive systems also may be used for example, in devices such as mixers, valves, compressors, clutches, and the like.
In prior art magnetically coupled devices, it is common to have a canister, otherwise known as a can, barrier, containment member, shell or sleeve, which separates an inner or driven magnet assembly and the fluid therearound, from an outer or drive magnet assembly. Such prior art canisters typically are a cylindrical, thin walled shell having a base with a flange which is connected or mounted between housing portions that may be referred to as a casing or housing body, and a housing head. Unfortunately, the fatigue life of prior art canisters is limited, in part due to cyclic hydraulic pressure loads to which the canister is subjected. Prior art canisters frequently are designed with thicker wall structures to handle such cyclic pressure loads. This unfortunately results in a less efficient magnetic coupling and greater heat generation. Also, if the canister is designed to support or hold an end of a shaft or pin, such as by use of a pin plate, it is common to require great care in the tolerances of such support or pin plate which is to be positioned within such prior art canister.
Accordingly, it is desirable to lengthen the fatigue life of a canister or containment member, or to otherwise make a canister less susceptible to the fatigue of common pressure fluctuations, while minimizing canister wall thickness. It also would be advantageous to reduce the burden of tolerances in manufacturing and to make an assembly for which the pin plate is more easily installed within a canister, and which is self-centering when the device is finally assembled.
The present invention addresses shortcomings in prior art magnetically coupled devices, while providing the above mentioned desirable features.